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FUELS (GENERAL METHODS)

303

represents the heat generated by the combustion of the fuel, and this,
divided by the weight of the fuel, gives the calorific power sought.

The capsule should be examined to ascertain if any appreciable quantity
of the coal remains unburnt. It is advisable to make several tests on each
sample, the highest result obtained, and not the mean, being regarded as
correct. It should be pointed out that, under the above conditions, coke
and anthracite burn with difficulty, while peat and many bituminous lignites
burn only incompletely.

In some Thompson calorimeters of English construction, the amount of
water placed in the glass cylinder weighs 29,010 grains (1879-85 grams). When
30 grains (1-944 grams) of the fuel are burnt, since 29010 ~ 30 = 967 = 537
X 9-r- 5, the rise in temperature in Fahrenheit degrees (increased by one-
tenth) indicates directly the grains of water at 100 transformable into steam
at 100 by the heat generated by one grain of the fuel, i.e., the evaporative
power calculated for water at 100 and not, as usual, at o.

It may be pointed out that, in England, calorific powers are mostly expressed
in terms of the British Thermal Unit (B.T.U.), which is the quantity of heat
necessary to raise the temperature of i Ib. of water (0-4536 kilo) by i Fahrenheit.
The large calorie  3-9683 B.T.U. and i B.T.U. = 0-252 large calorie. Further,
a calorific power of x calories per kilo corresponds with 1-8 x B.T.U. per pound,
or x B.T.U. per pound is equivalent to 0-5555 x calories per kilo.

With other Thompson calorimeters, the glass cylinder is marked at 2000
c.c. and also at 2148 c.c. (= 537 X 4). If the latter quantity of water is taken
and 2 grams of the fuel are used, the rise of temperature (increased by one-tenth),
multiplied by 2, will give directly the evaporative power (referred to water at
100 C. and therefore not to the standard usually adopted : see later).

(&) MAHLER BOMB CALORIMETER. This apparatus (Fig. 35), which is
among the best of those employed, consists of a vessel or bomb a of fairly
pure, forged mild steel, nickelled outside and enamelled inside : capacity
about 650 c.c., thickness of walls 8 mm., weight about 4 kilos. The bomb
is closed by a screwed iron lid b with lead packing and furnished in the
centre with a ferro-nickel conical screw valve r. The cover supports the
terminals, consisting of two platinum rods e, one passing through the cover
and insulated from it and the other fixed directly to the cover and supporting
a flat platinum dish c in which the fuel is placed. The two terminals are
connected by a small spiral of very thin iron wire which burns on passage
of the current (about 2 amps, at 810 volts) and so ignites the fuel in contact
with it.

The bomb rests on supports on the bottom of the brass calorimetric
vessel A, which contains 2,200 grams of water, a thermometer t divided
into fiftieths of a degree and allowing 0-01 to be estimated, and a spiral
stirrer d. To protect it from external influences, the calorimeter is placed
inside a double-walled metallic vessel B filled with water and covered with
felt.

To make a determination, exactly i. gram of the fuel, not too finely
powdered, is weighed into the capsule c and this placed in the bomb after
one of the iron wire spirals has been fitted to the terminals so that it comes
into contact with the fuel. The lid 6- is screwed tightly down, the valve
connected with a cylinder of compressed oxygen by means of a copper                                                                 19lyiique, II, p. 561. crown of the muffle.l and ether, dried at